Reduced resistance to sediment-trapping turfs with decline of native kelp and establishment of an exotic kelp
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Understanding the strength and type of interactions among species is vital to anticipate how ecosystems will respond to ongoing anthropogenic stressors. Here, we examine the ecological function of native (Ecklonia radiata) and invasive (Undaria pinnatifida) kelps in resisting shifts to sediment-trapping turf on reefs within the highly urbanized temperate Port Phillip Bay (PPB), Australia. Short-term (30 days) and long-term (232 days) manipulations demonstrated that kelp laminae can clear and maintain the substratum free of turfs, while conversely, removal of kelp leads to a proliferation of turfs. Analyses looking at the relationship between total length of E. radiata and U. pinnatifida and the area cleared of turf algae showed that the clearing effect of E. radiata over a year was greater than that of U. pinnatifida due to the annual die-back of the invasive. A natural experiment (608 days) identified that ongoing sea urchin (Heliocidaris erythrogramma) grazing led to native kelp bed decline, facilitating turf dominance. Even though U. pinnatifida establishes once native beds are disturbed, its ecological function in clearing turf is weaker than E. radiata, given its annual habit. In PPB, turfs represent the more persistent and problematic algal group and are likely changing the structure, function, and energy flows of shallow temperate reefs in this urbanised embayment.
KeywordsIntroduced seaweed Herbivory Sea urchin Reef degradation Novel alternative states
We thank S. Swearer for logistical support and M. Reeves, J. van Oosterom, D. Chamberlain, E. Fobert, and L. Barrett for invaluable assistance with fieldwork. This study was supported by a Victorian State Government, Department of Environment Land Water and Planning, Seagrass and Reefs program grant to CRJ and S. Swearer; a Holsworth Wildlife Research Endowment—Equity Trustees Charitable Foundation Grant to SER and SDL; and an Australian Postgraduate Awards to SER and NK. This manuscript has been greatly improved by the constructive review of two anonymous reviewers and editor D. Reed.
Author contribution statement
SER and SDL conceived and designed the experiments. SER, NK and SDL performed the experiments. SER analysed the data. SER, SDL and CRJ wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Statement of human and animal rights
This article does not contain any studies with human participants or animals performed by any of the authors.
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